1. Field of the Invention
The present invention relates to a method of driving a backlight, which drives a backlight that a liquid crystal display panel, etc. need to use therefor, a circuit for driving a backlight, to which this driving method has been applied, and an electronic apparatus in which this drive circuit has been incorporated.
2. Description of the Related Art
Among the liquid crystal image display apparatuses, there are some ones that need to use a backlight. For example, in the case of a small-sized image display apparatus that is used as a view finder which has been incorporated in a video camera, a planar fluorescent tube was used as the backlight. And by the fluorescence that occurs in this planar fluorescent tube, the liquid crystal panel was illuminated from a back surface thereof so that an image displayed on the liquid crystal panel might be visually recognized.
Here, the way of driving a conventional planar fluorescent tube when this tube has been used as a backlight will be explained below. Namely, from a video signal of the image displayed on the liquid crystal display panel, a horizontal synchronizing signal is separated to thereby produce a pulse signal whose period is synchronized with that of the horizontal synchronizing signal. The planar fluorescent tube was driven by the pulse signal with that horizontal period. Accordingly, by performing a pulse discharge using the pulse signal during the horizontal blanking period of the image displayed on the liquid crystal display panel, the planar fluorescent tube was fluoresced in a planar fashion. By doing so, fluorescence processing was performed in synchronism with the image displayed on the liquid crystal panel.
FIG. 1 is a view illustrating an example of a conventional pulse signal for driving a conventional backlight. A pulse signal whose period corresponds to the period of a horizontal frequency fH is continuously generated and that pulse signal is supplied to a drive circuit for driving a backlight. It was thereby arranged that fluorescence processing based on this pulse signal be performed. By performing such drive of the backlight using the horizontal blanking period, the cyclic period in which the backlight is fluoresced and the state in which the image is displayed are synchronized with each other. As a result of this, excellent display can be made which stands on the prevention of flickers, etc. of the displayed image that occur due to the failure of synchronization between the fluorescence of the backlight and the image.
Meanwhile, in the image display apparatus such as an electronic view finder or the like, it is preferable that the brightness of the image displayed on the display panel be able to be adjusted. In the case of the image display apparatus that has used the above-described backlight, it is possible to adjust the brightness of the image by changing the luminance of the fluorescence of the backlight. Here, in a case where there is made a construction wherein a pulse signal having the horizontal period such as that illustrated in FIG. 1 is supplied to the drive circuit for the backlight to thereby cause fluorescence thereof, it was carried out to change the pulse width PW of the pulse signal supplied to the drive circuit for the backlight in response to an adjusting value for adjusting the brightness at that time. Namely, when suppressing the luminance of the fluorescence of the backlight to be low, it was carried out to make narrow the pulse width PW of the respective pulse signal. While, when making the luminance of the fluorescence high, it was carried out to make wide the pulse width PW of the respective pulse signal.
However, when changing the brightness of the backlight by the above-described change of the pulse width, there was a limitation. That is, if making excessively narrow the pulse width PW of the respective pulse signal, the discharged state of the planar fluorescent tube inconveniently becomes insufficient to cause uniform fluorescence of the tube as a whole. Accordingly, in a case where adjusting the brightness of the backlight only by setting the pulse width, the range over which the brightness can be adjusted becomes inconveniently limited.
In order to adjust the brightness over a range that is wider than the adjustment range that can be realized only by this change of the pulse width, it is sufficient, for example, to decrease the frequency of the pulse signal for driving the planar fluorescent tube to xc2xd or ⅓ and to drive the planar fluorescent tube by the lower frequency pulse signal. By doing so, it becomes possible to perform the adjustment of the brightness beyond said adjustment range that uses the pulse width. However, because the frequency of the original horizontal synchronizing pulse is 15.75 kHz, in a case where having lowed the frequency of the pulse signal of 15.75 kHz to xc2xd or ⅓ thereof, this frequency becomes several kilohertz or so. Here, the band of several kilohertz or so is the one within which a human being can perceive the signal as a sound. Therefore, in a case where having driven the fluorescent tube constituting the backlight with a signal having a frequency of approximately 8 kHz, for example, which is the value that has been obtained by lowering to xc2xd, the sound with which the fluorescent tube and its drive circuit resonate at 8 kHz is inconveniently heard to the user. Therefore, the noises that are jarring to the ears are heard at all times. Accordingly, there was the problem that merely lowering the frequency of the pulse signal was unable to excellently adjust the brightness of the backlight.
The present invention has been made in view of the above-described problems and has an object to maintain an stable state of display of the image while performing the brightness adjustment of the backlight over a wide range without causing the generation of any noises.
To attain the above object, in a backlight-driving method of the present invention, it is arranged to thin a signal having a periodic waveform at every pseudo random period set in correspondence with an adjusting level of brightness, and to supply this thinned signal to an illuminating means for illuminating a back surface of a panel having displayed thereon an image as its drive signal.
According to this backlight-driving method, the thinned state of a signal that is thinned in correspondence with the brightness-adjusting level has a pseudo random period corresponding to the brightness-adjusting level. And, by this signal that has been thinned at this period or periods that are pseudo random, the illuminating means for the backlight is driven.
Also, a backlight-driving circuit of the present invention is equipped with a level-setting means for outputting a signal having a level corresponding to the adjusting level of brightness, a thinning means for thinning a signal having a periodic waveform at every pseudo random period that has been set in correspondence with an output level of the level-setting means, and a drive means for generating a backlight-driving signal based on an output signal of the thinning means.
According to this backlight-driving circuit, the thinned state of the signal that is thinned by the thinning means has a pseudo random period corresponding to the brightness-adjusting level. And, by this signal that has been thinned at this period or periods that are pseudo random, a backlight-driving signal is produced.
Also, an electronic appliance of the present invention is an appliance having a display panel having displayed thereon an image indicated by a video signal that has been input thereto and a backlight for use for this display panel, which is equipped with a level-setting means for outputting a signal having a level corresponding to the adjusting level of the brightness of an image displayed on the display panel, a thinning means for thinning a signal having a periodic waveform at every pseudo random period that has been set in correspondence with an output level of the level-setting means, and a drive means for generating a backlight-driving signal for use for the backlight according to an output signal of the thinning means.
According to this electronic apparatus, the thinned state of a signal that is thinned by the thinning means has a pseudo random period corresponding to the brightness-adjusting level. And, by this signal that has been thinned at this period or periods that are pseudo random, the backlight is driven to make its fluorescence.